Approximately 97 percent of water on the earth's surface is saline water, in view of the growing potable water consumption for domestic, agricultural or industrial applications, a need therefore exists to produce potable water from sources such as brackish water or seawater.
Different technologies permit to purify raw feed liquid, containing salt or other impurities, to produce potable water. The most commonly employed desalination techniques can be classified into two main categories, namely i) conventional thermal desalination such as Multi-Stage Flash (MSF), Multi-Effect Distillation (MED), Vapor compression (VC) and ii) non-thermal membrane based separation such as Reverse Osmosis (RO), Nanofiltration (NF), Forward Osmosis (FO), Electrodialysis Reversal (EDR), etc. Each individual technique has its own limitation ranging from low thermal efficiency such as Multi-Stage Flash (MSF) plants employing the flash evaporation process to Multi-Effect Distillation (MED) plants using spray evaporation of liquid feed to inefficient boron removal capabilities of non-thermal membrane based separation systems, such as RO, NF or EDR.
Membrane Distillation (MD) is a non-isothermal membrane separation process which employs hydrophobic membranes, first published in U.S. Pat. No. 3,361,645 A. With tremendous progress being directed towards membrane scientific research, efforts for industrial implementation of MD systems is currently gaining significant interest.
In brief, Membrane Distillation (MD) is a process which involves the evaporation of the heated liquid feed at the liquid/vapor interface located at the pores in a hydrophobic membrane. More specifically, evaporation of the liquid feed occurs only on the membrane pores and not in the bulk liquid feed. The rate of evaporation and vapor diffusion in MD is dependent on the transmembrane driving force, i.e. the difference in partial pressure of vapor across the membrane and the membrane permeability characteristic, which is defined based on the membrane geometrical parameters such as membrane pore size, membrane thickness, tortuosity and effective porosity. Therefore, a defined system configuration with a corresponding flow configuration such as temperature and feed flow rate will yield only a finite distillate flux in quantitative terms. Thermodynamically, one distinct characteristic of MD is a fall in the liquid feed temperature as evaporation progresses. This is defined as evaporative cooling, when sensible heat from the liquid feed is converted into latent heat during vaporization. The temperature drop of the liquid feed yields a lower driving force, resulting in a lower rate of vapor production and hence, a reduction in the global membrane distillate flux. The general limitations associated with low specific membrane flux, i.e. vapor/distillate produced per unit area of membrane and the closely inter-linked liquid feed temperature drop remain the most crucial disadvantages limiting the improving performance of the MD process for successful industrial implementation and commercialization. Tremendous efforts have been directed to capitalize on the MD process and various implementation techniques to increase the overall yield. This includes operating the MD system in partial vacuum to enhance liquid feed evaporation while reducing the vapor diffusion/transport resistance for enhanced condensation. Another solution includes heat recovery for partial reheating of the liquid feed to minimize the effect of a lower driving force arising from the liquid feed temperature drop. Reference is made to a recent peer reviewed journal paper related to a multi-effect MD apparatus by Zhao et al., ‘Experimental study of the memsys vacuum multi-effect-membrane-distillation (V-MEMD) module’, Desalination 323(2913), pp. 150-160 where those authors implemented a multiple heat recovery configuration via the integration of condensation foils and increased the membrane area to enhance the systemic yield of the apparatus. Although distillate production increased with an increased number of effects/stages and membrane area, two conclusions can be drawn from their experimental results. First, the specific flux was reduced from 3.8 to 3.0 l/m2-hr when the number of effects/stages was increased from 2 to 4 stages and second, a 55% reduction in specific flux from 8.7 to 3.9 l/m2-hr, was encountered when the number of membrane areas implemented in frames increased from 7 to 17 frames, i.e. 1.88 to 5.0 m2. The overall systemic yield and thermal efficiency was thus adversely lower when compared against conventional thermal desalination processes.
The relatively high energetic consumption and low efficiency yield of such an implementation is attributed to an inefficient condensation process, when said vapor produced in the preceding stage is transported via a conduit into the succeeding stage to be condensed, resulting in increased vapor transport resistance, which in turn results in a higher vapor chamber pressure, which in turn lowers the transmembrane driving force, and hence reduces distillate yield.
One objective of the present invention is to propose a membrane based high thermal efficiency water purification system having much lower thermal and electrical energy consumptions but also with an increased overall systemic membrane flux. Potential industrial applications for such a system include desalination, industrial processes, water treatment, shale oil water treatment, wastewater treatment or water recovery processes that require the removal of dissolved solids or impurities from any raw feed liquid via the thermal separation process.
To improve the thermal efficiency of an existing thermal separation process, for example desalination, one solution consists to efficiently preheat the raw feed liquid. Efficient preheating of the raw liquid feed can be accomplished via thermal energy recovered from the vapor condensation process occurring in the respective vapor sections, ideally, low vapor transport resistance will yield an optimum condensation process, which is not the case when vapor transmission conduits are present to channel vapor between preceding/succeeding modules to be condensed. More specifically, no specific gain in the specific membrane flux (L/m2-hr) can be achieved without towering the vapor transport resistance. For distilling systems capitalizing on the MD process, an increase in membrane area is used to accomplish this; that is, a large membrane surface area ensures sufficient vapor is produced to accommodate latent heat recovery for raw feed preheating and the reheating of the distilling liquid feed to minimize the temperature drop. This is not within the scope of the present invention as minimal membrane area and low vapor transported resistance via direct condensation are proposed in this present invention.
The present invention does not solely capitalize on the MD process, but a hybrid implementation of a combined flow boiling—MD process to increase distillate yield. The integration of heat exchanger boiling tubes into stages induces nucleate flow boiling on the external surfaces of the tubes in contact with the liquid feed located in boiling liquid sections. This nucleate flow boiling process occurring on the external surface of the tubes is technically dissimilar to processes such as spray falling film evaporation, flashing of vapor or evaporation at the liquid/vapor interface of a liquid, known already in other prior arts. The current implementation serves to augment the performance of such a distillation system by incorporating nucleate flow boiling heat exchanger tubes as the main vaporization process while complementing the MD process. This novel boiling feature and device, implemented in this current invention, will significantly increase vapor/distillate production and can yield up to a 95% increase of the total distillate, with the remaining distillate being produced via the MD process (essentially a few fold increase in capacity with respect to a MD only system). The scope of the invention widely diners from the state-of-the-art of MD systems in that it requires minimal membrane area but yet is capable of producing a higher distillate yield by primarily capitalizing on the combination of flow boiling on the external surfaces of the heat exchanger tubes within the boiling liquid sections and evaporation at the membrane surface via the MD process.
The second aspect of the invention involves the minimization of thermodynamic losses while significantly optimizing the heat and mass transport resistance across the distilling modules, resulting in a significant increase in the overall efficiency and performance of the distillation system. This is achieved by implementing direct vapor condensation via introduction of heat exchanger preheating tubes and/or internally flow boiling tubes within each immediate vapor section adjacent to the boiling liquid sections. This promotes simultaneous joint heat transfer processes involving, i) flew boiling on the heat exchanger tubes in the boiling liquid sections and ii) increased membrane evaporation via the MD process and iii) direct condensation on the external surface of the heat exchanger preheating tubes and/or internally flow boiling tubes in the vapor sections of the distilling units. Vapor transmission lines are thus eliminated by this present invention. The first advantage of the elimination of vapor transmission lines is prevention of oversaturation of the vapor sections, thus mitigating the rise in the vapor chamber pressure which would lead to lower transmembrane driving force. The direct condensation process, as proposed in this invention, results in a lower vapor chamber pressure, and thus enhances the MD process by maintaining a higher transmembrane driving force. In general, a 5% reduction in vapor chamber pressure will lead to a 5% increase in vapor production via the MD process. The second advantage is the elimination of vapor condensation in the transmission lines, which does not contribute to the heat recovery/vaporization process. For example, consider an 8 module distillation system with ideal efficiency; a loss of 1 kg of vapor due to condensation in the transmission line from the first module can result in a loss of 7 liters in distillate production from the succeeding distilling modules (assuming a liquid density, ρ of 1 kg/L), i.e. 1 liter of lost distillate production in each module from the 2nd to the 8th effect/stage. The third advantage involves lower thermal and mass transport resistance across the distilling modules, where the lower temperature drop is highly advantageous as this allows an increased number of effects/stages to be incorporated into the system which in turn reduces the specific energy (thermal and electrical) consumption of the distillation system.
Internal flow boiling tubes provides the additional flexibility to accommodate efficient direct condensation of vapor in the vapor chamber adjacent to the porous membrane. Phase change heat transfer, i.e. condensation and flow boiling, are known to be superior heat transfer processes due to their lower thermal resistances when compared to single-phase all liquid heat transfer. Five significant advantages can be derived with the integration of internal boiling heat exchanger tubes, namely, i) accommodating condensation of vapor on the external tube periphery of the said internal flow boiling tubes when said raw liquid feed temperature flowing internally within the preheating tubes has risen equal to the vapor temperature and no vapor condensation occurs, ii) extra vapor generation via internal flow boiling of the liquid feed flowing within the internally boiling tubes arising from the heat exchanger tube flow boiling, iii) enhanced condensation within the vapor chamber resulting in a lower vapor chamber, thus, a higher transmembrane pressure difference, iv) increasing the efficiency of the raw feed liquid preheating process when said hotter distillate condenses on the external surface of the internally flow boiling tubes flowing downwards due to gravity, preheating the raw liquid flowing inside the preheating tubes, and, v) accommodating a higher thermal energy load into the system. Under high thermal load conditions, when the said raw feed liquid temperature flowing in the preheating tubes is equal to the vapor temperature in the vapor chamber, and thus no additional vapor condensation on the preheating tubes is feasible, the internally flow boiling tubes will act as the heat sink by converting this additional latent heat arising from the vapor condensation process to latent heat of vaporization via flow boiling. For example, consider an 8 module distillation system with ideal efficiency. An additional 1 kW of thermal energy consumed to generate vapor via the flow boiling process in the boiling liquid section of the first effect can be effectively consumed via the internal flow boiling process by the internal flow boiling tubes inside the vapor section of the first effect. This results in an additional eight times gain in vapor/distillate produced by the 8 module distillation system (distillate production equivalent to 8 kW of thermal energy assuming ideal efficiency) This performance enhancement is technically not achievable with the integration of solely the preheating tubes. A performance comparison simulation is presented in the later section of this disclosure.
EP 0088315 A1 discloses a desalination device and process comprising a spiral wound air gap membrane distillation device with heat recovery for raw feed liquid preheating. In this prior art, a spirally elongated vapor permeable membrane separates the feed liquid from the elongated vapor chamber white a vapor impermeable layer acting as a condensing sheet separates the condensing vapor from the raw feed liquid to be preheated. Internal raw feed liquid preheating is accomplished using thermal energy transferred from the condensing vapor. The condensed distillate then flows toward a distillate outlet located in the downstream direction of the hot feed liquid flow. In this embodiment, a single evaporation and condensation process is disclosed as opposed to multiple vaporization and condensation process presented in the current invention. This invention capitalizes primarily on the membrane distillation (MD) principle as opposed to the current combined processes of heat exchanger tube flow boiling and the MD principle in the current invention.
WO 2005/089914 A1 discloses a single spiral wound and multi-stage membrane distillation device and method with raw feed liquid preheating solution being accomplished externally using thermal energy from the condensing vapor, brine liquid and liquid distillate. In the plurality membrane distillation apparatus embodiment, the vapor produced in the evaporator and subsequent stages is channeled via a conduit from one stage to another stage for feed liquid reheat. One of the disadvantages is increased vapor transport resistance and transmission losses. The loss of vapor in the transmission conduit due to condensation within the conduit will result in a non-efficient heat recovery process for the feed liquid reheat and an overall loss in distillate yield. This invention capitalizes primarily on the membrane distillation (MD) principle as opposed to the current fully integrated heat exchanger tube flow boiling and MD processes presented in the current invention. DE 102009020179 discloses a multi-stage membrane distillation apparatus having an evaporator and multiple condensation/evaporation stages. Raw feed liquid preheating is accomplished via an external thermal energy source. This solution does not implement any integrated compact raw feed liquid preheating configuration in the vapor chamber, which is the objective of the present invention. This invention capitalizes primarily on the membrane distillation (MD) principle as opposed to the current fully integrated processes of heat exchanger tube flow boiling and MD processes presented in the current invention.
Another example, US 2014/0216916 A1 discloses a membrane distillation device for the purification of a feed liquid, comprising a plurality of condensation/evaporation stages, in which the raw feed liquid is preheated in at least one additional vapor chamber, to which the vapor fed to one condensation/evaporation stage is supplied and in which the vapor is condensed. In this disclosure, the vapor produced is again channeled via a conduit from one stage to another stage. This incurs additional vapor transport resistance and, thus, can result in an increase in the vapor chamber pressure and as a consequence, results in a less efficient vaporization process. In all the embodiments, this invention implements vapor transmission lines/conduits to accommodate raw liquid feed preheating and reheating of the liquid feed. As clearly indicated in the text description and figures, heat recovery for feed preheating is facilitated via an enlarged condensation/evaporation stage at the rear end of each distilling module to accommodate the preheating device, which is separated from the evaporation and condensation unit. Furthermore, this solution does not implement an integrated compact direct feed preheating configuration that minimizes heat/mass transport resistance, which is another objective of the present invention. Heat exchanger flow boiling tubes are not implemented in the liquid section of the modules to generate vapor and preheating tubes are not being implemented within the vapor chamber directly adjacent to the porous membranes. This invention also does not implement any internal flow boiling tubes within the vapor chambers. More specifically, this invention capitalizes primarily on the membrane distillation (MD) principle as opposed to the integrated heat exchanger tube flow boiling and MD processes presented in the current invention. Thus, one notes that there are no flow boiling tube devices to generate vapor, while the current invention implements this process.
WO 2014/020461 A1 discloses a desalination system comprising a steam raising device, a membrane distillation device and a heat exchange device, wherein the liquid fed into the heat exchange device is heated by the brine liquid from the steam raising device. In this prior art, the vapor produced in the steam raising device is channeled via a conduit into another membrane distillation module. Hence, this invention implements vapor transmission lines/conduits to accommodate vapor condensation. This causes increased heat and mass transfer resistance, resulting in a less efficient condensation/evaporation process. Clearly, this invention relates to the conventional MD principle whereby the flow process is modified to introduce partial liquid feed boiling on the non-permeable condensation walls via heat recovery from the liquid brine and condensing vapor. Notably, heat exchanger flow boiling tubes are not being implemented in the liquid section of the modules to generate vapor and preheating tubes are not being implemented within the vapor chamber directly adjacent to the porous membranes. This invention also does not implement any internal flow boiling tubes within the vapor chambers, which is the scope of this current invention. Furthermore, the vapor condensation and preheating process is accomplished by means of external heat exchangers resulting in a non-compact structure for the system.
US 2010/0072135 A1 discloses a membrane distillation method (or the purification of a liquid. This invention does not implement any tubular flow boiling process to increase distillate yield; distillate yield is achieved only via the membrane distillation process. There are no integrated flow boiling devices to induce boiling (hence additional vapor production) within the distillation apparatus. Additionally, no internally flow boiling tubes are implemented in the system. The distillate yield for this invention depends entirely on the membrane area whereby higher yield will require additional membrane area. In the current invention, the primary process is accomplished via heat exchanger tube flow boiling while MD is only identified as the secondary process.
EP 2 606 953 A1 discloses a membrane distillation system comprising a plurality of membrane distillation modules that are pressure wise coupled in series and that where distillate exit is provided with a fluid permeable hydrophilic membrane, wherein the fluid permeable membrane is an elastomeric valve. The disclosure claims the membrane distillation unit operated at the lowest pressure is coupled to the distillate collector unit via a siphon and that a heat exchanger is present between the distillate collector, the first of said fluid-permeable membrane and that a plurality of inputs coupled with said distillate exits are extended with a heat exchanger along the said connection preferably in a counter flow direction. The inventors also claim that the membrane distillation system has a steam riser circuit comprised of a heat exchanger where the fluid in the steam riser circuit is heated via rest heat from other systems. Therefore, this invention does not describe a heat exchanger tube flow boiling process, using preheating tubes for raw feed preheating, and internally flow boiling tubes inside the vapor chambers.
WO 2014/163507 A1 relates to a membrane distillation system and an energy source to provide heat for the MD process, wherein the energy source originates from the generator and is transferred via an intermediate cycle to the first distillation module. Fluid feed is heated up via a heat exchanger and optionally partially evaporating the fluid in the circuit to produce a two-phase food mixture before channeling the said liquid into the first membrane module. This invention capitalizes on the membrane distillation process including the preference of introducing two-phase feed into the membrane modules. Therefore, contrary to the current invention. WO 2014/163507 A1 does not disclose a distillation module with hybrid features comprising devices such as integrated flow boiling tubes in the liquid feed section, preheating and internally flow boiling tubes in the vapor chambers.
WO 2014/058305 A1 discloses a membrane distillation system and the method of starting up such a system and the use thereof. This invention relates to externally generating a multi-phase feed using a pretreatment module before channeling the said feed into a steam generator. This invention does not propose a distillation module with hybrid features comprising devices such as integrated flow boiling tubes inside the liquid feed section, preheating and internally flow boiling tubes in the vapor chambers.